1. Field of the Invention
The present invention relates to the manufacture of semiconductor devices and, in particular, to the manufacture of bipolar transistors having a polysilicon emitter, which comprise reduced emitter resistance.
2. Description of Prior Art
In bipolar transistors, which are designed to take high powers and speeds, use is already made of polysilicon emitters. In this connection, with respect to the theoretical and experimental aspects of the use of bipolar transistors having a polysilicon emitter, reference is made to the article by C. R. Selvakumar, “Theoretical and Experimental Aspects of Polysilicon Emitter Bipolar Transistors”, published in IEEE Press, 1988, pages 3 to 16.
Therefore, one embodiment of a bipolar transistor having a polysilicon emitter provides a heavily doped polysilicon layer located above the basis, which serves both as a diffusion source for the generation of a flat (emitter/basis) semiconductor transition and as a means for contacting the flat emitter region. After performing the conventional processing steps for manufacturing the basis region and the emitter window openings, either non-doped or doped polysilicon will be applied, into which, if the polysilicon is non-doped, an exact quantity of arsenic atoms will be implanted. Thereupon, by way of heat treatment (tempering), damages will be annealed, and the emitter/basis semiconductor transition is formed.
As can be seen from the above-cited article on page 4, one of the critical processing steps in the manufacture of bipolar transistors having polysilicon emitters consists of the treatment of the waver exactly before applying the polysilicon. Therefore, the many different treatment methods, as are known in the state of the art, may be roughly sub-divided into two categories. The first treatment refers to intentional or unintentional growth of a thin oxide layer (0.2 to 2 nm). The second treatment refers to the epitaxial growth of a thin thermal nitride layer (approximately 1.0 to 1.5 nm). The “interface” treatment is important, since the same has strong effects on the electrical characteristics of bipolar transistors having a polysilicon emitter.
As has been mentioned above in brief, it is tried to achieve bipolar transistors having high cut-off frequencies and high current gains by forming the emitter(s) of a bipolar transistor by depositing a heavily doped polysilicon layer. The doping agent in the polysilicon layer will then diffuse, by way of tempering, from the polysilicon layer into the single crystal silicon substrate below, where it forms the electrically-active emitter area of the bipolar transistor. Here, the polysilicon used serves as a doping agent source, as a feed and also as a landing surface for the contact terminal holes yet to be formed. As for the operational properties of the transistor, the use of polysilicon has the following decisive advantage that the interface between the polysilicon layer and the single crystal silicon substrate serves as a diffusion barrier for minority carriers which are injected from the basis, thus clearly increasing current gain and cut-off frequency of the transistor.
However, one disadvantage of polysilicon is the specific resistance which is by orders of magnitude higher as compared to metals. The relatively high emitter resistance resulting therefrom especially affects the high-frequency properties of the bipolar transistors. Owing to these problems, it has been tried to use as thin a polysilicon layer as possible. On the other hand, a certain minimum thickness of mostly far above 100 nm is required, since an etching of contacting holes for the contact pads has to stop on this polysilicon layer in order to ensure the process safety during the manufacture of bipolar transistors. The problem concerning the emitter resistance still increases with modern bipolar transistors having very narrow emitter windows, since the polysilicon used may completely fill the emitter window in this case, and, thus, the height of the polysilicon layer over the active emitter further increase
It should be appreciated that, instead of polysilicon, an amorphous silicon may also be used which, in turn, may rest to crystallize in subsequent tempering processes.
In order to solve the problems shown above concerning the manufacture of bipolar transistors having a polysilicon emitter, concepts haven been taken up, which provide a thermal silicidation of the emitter after depositing a metal layer. Silicides are metal/silicon compounds, which are used in silicon technology as temperature-stable, low-resistance traces and contacts. The silicide layers typically provide a thickness of 0.1 to 0.2 μm. However, the silicide layer formed in this manner is, as a rule, relatively irregular, thus, in practice, making it impossible to fill the emitter window with this layer.
As a further measure, the layer thickness of the polysilicon was kept as low as possible and the doping of same was kept as high as possible. If possible, filling the emitter window with polysilicon was avoided, which, however, in earlier technologies, was much easier owing to larger emitter dimensions. If, after depositing the polysilicon on the emitter, a very narrow gap is left, increased efforts were necessary, depending on the technology chosen, when etching the contacting holes, since this gap may be filled with an undesired material, e.g. with a nitride barrier, when depositing further layers.
In many cases, the negative influence of the emitter resistor on the transistor properties was simply accepted and/or it was tried to compensate for this negative influence in terms of circuit technology.